Telecommunications systems such as UMTS and LTE can use licensed carrier spectrum (e.g., the 700 MHz spectrum) for communicating with user equipments (UEs). The 3GPP initiative and others have discussed the use of unlicensed carrier spectrum (e.g. the 5 GHz spectrum) for LTE, through a technology referred to as “License Assisted Access” (LAA) or “LTE in unlicensed spectrum” (LTE-U). LAA intends to allow LTE equipment to operate in both a licensed carrier spectrum (“licensed carrier”) and an unlicensed carrier spectrum (“unlicensed carrier”).
The unlicensed carrier can be used as a complement to the licensed carrier. The licensed carrier can be used to establish a primary cell (PCell), and the unlicensed carrier can be used as a secondary cell (SCell). Devices connect in the licensed carrier (in the PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed carrier (in the SCell). LTE allocates different time periods, called subframes, for the transmission of different units of data in the licensed carrier, and the same subframe timing in the licensed carrier can be used for transmission in the unlicensed carrier.
The unlicensed, 5 GHz carrier is used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.” To coexist with the Wi-Fi system, a transmission by a LTE device on the SCell may need to conform to a “Listen Before Talking” (LBT) protocol to avoid collisions and causing severe interference to on-going transmissions, by performing channel sensing before attempting to transmit on the SCell. Different regions may limit the maximum duration of a transmission burst in the unlicensed carrier.
LTE transmission by a UE in the unlicensed carrier may be scheduled by a scheduling node, such as a eNB serving the UE. The scheduling may use self-scheduling or cross-carrier scheduling. In case of self-scheduling, an eNB first starts the LBT operation and measures the interference level in the unlicensed carrier to determine if the carrier is free or occupied. If the LBT procedure indicates the unlicensed carrier is free, the eNB transmits, via the unlicensed carrier and to the UE, control signaling containing a UL grant message, which grants the UE a UL subframe in the unlicensed carrier that starts after a delay period d; otherwise the eNB refrains from sending the grant to the UE. Because of the LBT operation, the transmission of the UL grant message may be delayed by the amount of time that is needed for the eNB to carry out the LBT operation.
In case of cross-carrier scheduling, the eNB transmits, to the UE, the UL grant message over the licensed carrier. The UL grant message that is transmitted over the licensed carrier grants to the UE a UL subframe in the unlicensed carrier that begins after a delay d. Because the UL grant message is transmitted over the licensed carrier, the eNB does not perform any LBT operation before transmitting scheduling signals over the licensed carrier.